Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Hidden fault may contribute to Bay Area earthquake risk

17.12.2004


Figure 1. Tectonic setting of region of proposed study. Generalized location of major fault strands through the San Francisco Bay region are shown by dashed lines. Primary study area is outlined by bos. Hypothetical blind thrusts are schematically indicated by ’dashed’ thrust fault symbols - are purely schematic at present. Mt. Tamalpais and Bolinas Ridge are identified
Credit: Penn State, Kevin Furlong


Earthquakes are not unusual in the San Francisco Bay Area, but a team of Penn State geoscientists believes that the hazard may be greater than previously thought because of a hidden fault under Marin County.

"We think we have evidence that there is an additional earthquake hazard in the San Francisco area due to a blind thrust fault," says Dr. Kevin P. Furlong, professor of geosciences. "Blind thrust faults are notorious because they are hard to find until an earthquake occurs on them. A blind thrust fault caused the 1994 Northridge earthquake."

The San Francisco Bay Area has a variety of known faults running through it. The San Andreas fault runs on the west, while the Hayward fault is on the east and shifts into the Rodgers Creek fault northeast of the city. The San Gregorio fault, west of the San Andreas, meets that fault near the Golden Gate Bridge.



The slip rate on the Hayward fault is about 9 millimeters a year and the slip on the Rodgers Creek fault is probably 6 to 9 millimeters a year, according to Furlong. These two rates are consistent. However, the slip on the San Andreas fault south of the Golden Gate Bridge is 17 millimeters per year and the slip rate north of the bridge is 24 millimeters per year. Adding in the San Gregorio fault slip of about 3 millimeters a year, the slip rate on the northern San Andreas is still inconsistent with the southern portion of the fault by as much as 4 millimeters per year.

Compounding the geological confusion is the existence of Mt. Tamalpais at 2,640 feet sitting northeast of San Francisco in Marin County. Geologists are unsure why Mt. Tamalpais is there and what formed it.

Furlong and Dr. Eric Kirby, assistant professor of geosciences, looked for an explanation for the different slip rates on the San Andreas fault north and south of the Golden Gate that would also explain how Mt Tamalpais came to rise above the bay.

"In the past, the thought was that there must be something wrong with the calculations on the San Gregorio fault," says Furlong. "That is, the slip rate must be closer to 7 millimeters per year because we do not see any other faults."

The researchers hypothesized that a fault running diagonally from the northern Hayward fault to the San Andreas in Marin County could transfer the necessary motion to the San Andreas and might explain high topography around Mt Tamalpais. However, proving the fault’s existence turns out to be a difficult task.

Thrust faults occur when one piece of terrain rides up over another forming a characteristic uplift pattern with one side gradually sloping up and the other more precipitous. Blind thrust faults terminate below the Earth’s surface and are therefore blind.

"We asked, can we demonstrate that Mt Tamalpais is currently going up, thrusting?" said Furlong. "Uplift rates are just at the limit of what can be measured with geographic positioning systems and there are not sufficient prior measurements on the mountain to compare with anyway," he told attendees at the fall conference of the American Geophysical Union in San Francisco.

Estimating that the uplift is about 1 millimeter per year – less than normally observable with GPS, the researchers looked for another way to measure uplift. They considered the topography of Marin County. Bolinas Ridge – just east of Point Reyes – runs up the west side of the area just to the west of Mt. Tamalpais and has a gradually rising slope, similar to the uplift predicted by a blind fault. It also has numerous streams running off the ridge peak into the San Andreas fault.

"If an area is uplifting, then we typically see steep rivers, but if the uplift is slow or nonexistent, then we see gentle rivers," says Furlong. "What we find is that the rivers become substantially steeper in the southern portion of Bolinas Ridge, implying that there may be active uplift in the area."

The rock making up Bolinas Ridge is of uniform composition, so differences in river slopes probably reflect differences in erosion rate. Assuming there is a blind thrust fault, the slip rate on the fault would need to be 3 to 4 millimeters to even out the discrepancies north and south of the Golden Gate Bridge. If the slip is 3 to 4 millimeters, then the researchers calculate that there would be potential for an earthquake of magnitude 6 to 6.5 on the fault occurring on a time scale of several hundred years.

Courtney B. Johnson, graduate student in Penn State’s department of geosciences, is presenting a poster that further explores the size and earthquake potential of the blind thrust fault.

"No large earthquakes have occurred in that area for at least the 150 to 200 years we have records," says Furlong. "In fact, there are very few earthquakes of any size in this general area."

The researchers are planning research to prove that the blind thrust fault exists. Unfortunately, standard seismic imaging will not work on the type of rock in the area, so alternative methods are necessary.

"The Loma Prieta earthquake did a lot of damage to the East Bay and Marina areas of San Francisco," says Furlong. "An earthquake in Marin County, while smaller, is much closer and is of concern not only to those in the county, but also to that part of San Francisco."

A’ndrea Elyse Messer | EurekAlert!
Further information:
http://www.psu.edu

More articles from Earth Sciences:

nachricht Six-decade-old space mystery solved with shoebox-sized satellite called a CubeSat
15.12.2017 | National Science Foundation

nachricht NSF-funded researchers find that ice sheet is dynamic and has repeatedly grown and shrunk
15.12.2017 | National Science Foundation

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First-of-its-kind chemical oscillator offers new level of molecular control

DNA molecules that follow specific instructions could offer more precise molecular control of synthetic chemical systems, a discovery that opens the door for engineers to create molecular machines with new and complex behaviors.

Researchers have created chemical amplifiers and a chemical oscillator using a systematic method that has the potential to embed sophisticated circuit...

Im Focus: Long-lived storage of a photonic qubit for worldwide teleportation

MPQ scientists achieve long storage times for photonic quantum bits which break the lower bound for direct teleportation in a global quantum network.

Concerning the development of quantum memories for the realization of global quantum networks, scientists of the Quantum Dynamics Division led by Professor...

Im Focus: Electromagnetic water cloak eliminates drag and wake

Detailed calculations show water cloaks are feasible with today's technology

Researchers have developed a water cloaking concept based on electromagnetic forces that could eliminate an object's wake, greatly reducing its drag while...

Im Focus: Scientists channel graphene to understand filtration and ion transport into cells

Tiny pores at a cell's entryway act as miniature bouncers, letting in some electrically charged atoms--ions--but blocking others. Operating as exquisitely sensitive filters, these "ion channels" play a critical role in biological functions such as muscle contraction and the firing of brain cells.

To rapidly transport the right ions through the cell membrane, the tiny channels rely on a complex interplay between the ions and surrounding molecules,...

Im Focus: Towards data storage at the single molecule level

The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.

Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

See, understand and experience the work of the future

11.12.2017 | Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

AKL’18: The opportunities and challenges of digitalization in the laser industry

07.12.2017 | Event News

 
Latest News

Engineers program tiny robots to move, think like insects

15.12.2017 | Power and Electrical Engineering

One in 5 materials chemistry papers may be wrong, study suggests

15.12.2017 | Materials Sciences

New antbird species discovered in Peru by LSU ornithologists

15.12.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>